The Moon continues to be a mystery for scientists, but a new study gives convincing evidence to why one side of the moon is mountainous and one side is low and flat.

We know the moon, but in some ways we know surprisingly little about it. New science is emerging about its core, for instance, and new reports come out constantly about how much water is inside our natural satellite. In a new study out today, scientists try to answer another mystery of our natural satellite: Why that one side we do see is significantly flatter and more volcanically active than the mountainous "dark side of the moon."

Most Popular

For research that appears in Nature, Erik Asphaug and Martin Jutzi of the University of California Santa Cruz put together a simulation suggesting that billions of years ago, there was a smaller, co-orbiting moon that crashed into the moon that we know. But that collision wasn't a flashy cosmic event, the researchers say. In fact, it was so slow they call it the "big splat."

"The moons are both made of rock and if you have something impacting it at 2 km per second, it's not going to make a crater," Asphaug says. "It's just going to crush and deform. So you see the thing 'splatting.'" A faster collision would have sent out shock waves and created a crater. But because this collision was only 2 km per second, the debris from the object piled onto the moon and created an extra layer of crust on the far side, which is why it's strikingly different from the near side of the moon, according to the team.

Asphaug and Jutzi hypothesize that the "big splat" happened roughly 4.4 billion years ago, just 100 million years after the moon we have today was formed by a Mars-sized object smacking into the early Earth and jettisoning the material that become the moon. Lots of other junk was floating around the inner solar system back then, and some of it would have also entered the Earth's orbit where it could have grazed the moon.

"We think that the object was a co-orbital with the moon," Asphaug says. "So why would these things collide? The moon is receding from the earth at about 10 centimeters per century. The farther the moon is, the less influence it has and the sun starts to matter more." That is, the fact that the moon is receding away from the Earth affected the trajectory of the two moons, and they ultimately collided. Nearly all the smaller moon's material was absorbed and added onto the crust of our moon, and what remained of the smaller moon careened on its way through the early solar system.

The "big splat" could explain even more about the moon than the extra material on the far side, the researchers say. The force and direction of the impact also played a part in the distribution of potassium and rare earth elements (also known as KREEP elements), Asphaug says. What would normally be an even distribution of these elements turned out to be biased heavily toward one side of the moon, the near side. "If you look at lunar rocks [which were extracted from the near side], they're rich in KREEP elements," he says. "It's like all the jelly got squeezed out to the other side, since we don't see any KREEP material on the far side (with explainable exceptions)." Finally, Asphaug says, the simulation shows how a two-moon collision could have set the stage for the volcanic activity we see on the near side of the moon: All the escaping heat from the smash-up headed that way.

While there have been other models addressing the formation of the highlands and mountains on the moon, including Asphaug's and Jutzi's colleagues at UCSC, their "big splat" model is the first of its kind, they say. "It's a new kind of collision physics. We haven't really studied this before," Asphaug says.